1. Field of the Invention
The present invention relates to an image heating apparatus preferably used for a heat-fixing device mounted on an image forming apparatus such as a copying machine and a printer, and a pressure roller used for the image heating apparatus.
2. Related Background Art
Recently, in a business machine field, a product having small electric power consumption has been demanded. As for an image forming apparatus such as a copying machine employing an electrophotographic system or a laser beam printer, an attempt of reducing heat capacity of a heat-fixing device has been made in order to suppress electric power consumption. As an on-demand heat-fixing device, a ceramic heater type device, an electromagnetic induction type apparatus, or the like has been practically used. The ceramic heater type device includes a ceramic heater arranged in a film-shaped rotary member, and a pressure roller cooperating with the ceramic heater through the film-shaped rotary member to constitute a heating nip portion, and heats an image on a recording material with heat of the ceramic heater while transporting the recording material in the heating nip portion. According to the electromagnetic induction type device, a film-shaped rotary member or a fixing roller generates heat by itself.
In the above-mentioned background, as a result of further progress of shortening in a so-called first print time and energy saving, shortening of rising time of a heating operation and reduction of electric power consumption of a fixing device have been especially demanded.
Therefore, “heat insulating property” has been recently desired as an especially important function to be required for the pressure roller used in the heat-fixing device.
This is based on a concept which reduces heat conductivity of an elastic layer of the pressure roller so as to suppress quantity of heat taken away from a heating member by the pressure roller at the time of starting the operation of the heat-fixing device, thereby improving a temperature increasing rate of the film-shaped rotary member or the fixing roller in contact with the pressure roller.
Therefore, it has become the most important issue to use a material having low heat conductivity for a heat resistant elastic layer constituting the pressure roller.
An example of a material achieving low heat conductivity of the heat resistant elastic layer includes silicone rubber foam utilizing low heat conductivity of gas.
Furthermore, a pressure roller having excellent heat insulating property, which contains a hollow filler in an elastic layer thereof, is proposed, for example, in Japanese Patent Application Laid-open No. 09-114281.
In addition, a pressure roller containing a resin microballoon in an elastic layer has been already proposed in Japanese Patent Application Laid-open No. 2000-143986.
However, while such a pressure roller is capable of achieving reduction of heat conductivity, it simultaneously has the following problems.
For example, a method of adding a heat decomposition type foaming agent to silicone rubber, and a method of generating a foam using hydrogen gas as a foaming agent which is a by-product at the time of curing are known as a method of producing silicone rubber foam utilizing low heat conductivity of gas. These methods have difficulty of forming a finely and evenly foamed cell. As a result, since surface smoothness of the foam is insufficient, there arises a problem in that the pressure roller is contaminated by a toner.
Here, the surface smoothness and the toner contamination of the pressure roller will be described in detail. In general, a mold releasing layer (e.g., a fluororesin tube or a fluororesin coating) is provided on an outer peripheral surface of the elastic layer in order to prevent the toner contamination of the pressure roller. Since thickness of the mold releasing layer is approximately several tens of μm, the surface smoothness of the roller depends on smoothness of the elastic layer. If there exist convex and concave portions on the surface of the elastic layer, which forms convex and concave portions on the surface of a surface releasing layer. As a result, a contaminant toner is deposited on the concave portion of the surface releasing layer. Therefore, it is preferred that the elastic layer has a sufficient surface smoothness.
The applicant of the present invention has already proposed that hardness of a minute area on the mold releasing layer surface is a factor related to toner contamination of the pressure roller and that the hardness is preferably low. In other words, it is not effective to increase the thickness of the mold releasing layer as a technique for improving the surface smoothness because the toner contamination of the pressure roller would be increased unwillingly.
As for a method of adding the hollow filler into the silicone rubber, the hollow filler reduces heat conductivity by providing a gas portion to a cured product like sponge rubber. Therefore, it is possible to improve the surface smoothness by using a hollow filler having a small particle diameter.
In the case where an inorganic hard filler is used as a hollow filler, if the hard filler is added in such an adjusted amount as to enable desired reduction of a heat conductivity, a hardness of the pressure roller becomes excessively large. As a result, a fixing nip width with which a satisfactory fixing property is achieved cannot be obtained.
In the case where a hollow filler having elasticity by itself (i.e., a resin hollow balloon) is used, the resin hollow balloon is broken during use (endurance). As a result, problems such as compression set and reduction of hardness during endurance would be caused.
In consideration of the above-mentioned problems, a method of producing a foam, which enables a finely foamed cell without using a hollow filler, is disclosed in Japanese Patent Application Laid-open No. 2002-114860.
This method includes mixing a water-absorbing polymer containing water in silicone rubber and evaporating a water content at the time of heat-curing the rubber so as to form a foamed cell (bubble) in a silicone rubber elastic layer (hereinafter, the method is referred to as “water evaporation foaming method”). This method has advantages that a foamed cell size can be controlled by varying a particle diameter of the water-absorbing polymer in a powder form and a content of the water, so that a fine cell can be obtained.
A pressure roller obtained by the water evaporation foaming method exhibits an extremely high open-cell rate, in the case of suppressing the heat conductivity of the elastic layer, although it depends on a blending amount of a water-absorbing polymer. In the case of a foam using a water-absorbing polymer, since a cell is formed by evaporation of the water content in a heat-curing process, the cell in the obtained foam does not have a wall such as that of a hollow filler. Since the cell itself does not have a wall, an increase of the blending amount of the water-absorbing polymer results in that the cells after heat-curing are coupled to each other to have an open-cell property. If an expansion ratio of the foam is increased in order to further reduce heat conductivity, a border portion amount of the foamed cells becomes thin and the foam exhibits a high open-cell rate, thereby causing a functional deterioration such as deterioration of impact resilience which is inherent in rubber.
In the case where a low heat capacity film unit is used as a unit opposing a pressure roller, in order to simplify an apparatus structure, only a regulating member mounted on each of left and right sides of a fixing film regulates displacement of the fixing film in a left or right direction. As a result, if a pressure roller having deteriorated impact resilience (i.e., grip) is used, the regulating property for suppressing the displacement of the fixing film in the left or right direction also deteriorates. As a result, the rupture of the end portion of the fixing film of the item (2) would be caused.